Ink jet printers have come into widespread use because they can print high quality color images at reasonably high speeds. Such a printer usually comprises a rotary drum for supporting a sheet of paper or other recording medium and a print head which is spaced from the drum surface and moved parallel to the drum axis. The movements of the drum and head are coordinated so that the head scans a raster on the drum surface every rotation of the drum. The print head includes one or more ink nozzles (one for each color ink), each of which can direct a jet of ink droplets to the paper on the drum. The jetters are activated at selected positions in the scan to print an image on the paper composed of an array of ink dots.
Ink jet printing systems can be divided into drop-on-demand and continuous jet systems. In the former, the volume of a pressure chamber filled with ink is suddenly decreased by the impression of an electrical driving pulse whereby an ink droplet is jetted from a nozzle communicating with that chamber. Thus, a single drop of ink is transferred to the paper or other recording medium by a single driving pulse following which the system returns to its original state. During printing, a succession of such droplets is ejected as a jet in response to a succession of drive pulses to print an image on the paper according to a predetermined dot matrix. In the continuous jet-type system, a succession of ink drops is ejected from a jetter or nozzle. Selected ones of these drops are deflected electrostatically into a gutter; the remaining undeflected drops reach the paper on the drum and form the printed image thereon according to a standard dot matrix. While the present invention is applicable to both jet printer types, we will describe the invention primarily as it is applied to a continuous jet-type printer.
Ink jet printers are inherently capable of high speed, high resolution color printing. However, this requires precise manufacture and assembly of the component parts of the printer. Even then, the printer will not print with all colors in proper register unless the printer is calibrated so that the various nozzles on the print head are positioned properly relative to the drum and relative to each other.
In other words, the positions of the printed dots in the direction along the drum (X axis) must be referenced to the home position of the print head. In addition, various nozzles on the print head must be aimed (in yaw) and their actuations timed so that the ink dots produced by all the nozzles at the same dot position in the scan will be in X axis alignment.
The positions of the dots in the direction around the drum are not controlled by aiming the nozzles. Rather, such control is achieved electronically by controlling the timing of the control signals that fire the jets in relation to the instantaneous position or phase angle of the drum. When the printer is calibrated properly both mechanically and electronically, the different color ink dots produced by the nozzles at a given dot position in the raster scan will be superimposed to form a single well-defined ink dot of a selected, usually subtractive, color.
Conventionally, in prior printers of this general type, the mechanical aspects of the calibration procedure have been carried out by an operator observing the dots printed on the paper or other recording medium wrapped around the drum and manually adjusting the yaws of the nozzles on the print head and the timing of the jets so that the dots printed by the various nozzles are in superposition at each dot position in the raster scan. Such manual calibration is a tedious and time-consuming trial and error procedure. Not only must it be performed at the factory when each printer is manufactured, but also, it must be done whenever any maintenance is performed on the printer which effects the positions of the ink dots. For example, the printer must be recalibrated whenever a nozzle is replaced or whenever there is relative movement of the nozzle and its knife edge. It would be desirable, therefore, if means existed on the printer itself for executing the calibration procedure automatically because this would result in considerable monetary savings both in terms of operator time and downtime of the printer.